Finally, [1]} suggested that compact elliptical galaxies (CEGs) surrounded by an X-ray halo should be considered as FGs. In fact, detailed X-ray observations of two of these systems found that mass and entropy profiles and concentration are compatible with other FGs studied in the literature. In this case, the progenitors are expected to be the so-called “red-nuggets”, compact galaxies found a \(z \sim 2\)  [2]}. This case is indeed peculiar: the authors did not study FGs in order to find their progenitors, but better suggested that a new type of galaxy should be considered in the fossil category.
